FILE - Paul Whelan, a former U.S. Marine, who was arrested in Moscow at the end of last year, waits for a hearing in a court in Moscow, Russia, May 24, 2019.
FILE - Paul Whelan, a former U.S. Marine, who was arrested in Moscow at the end of last year, waits for a hearing in a court in Moscow, Russia, May 24, 2019.

The United States has formally protested what it is calling Moscow's mistreatment of Paul Whelan, a former U.S. Marine who has been detained in Russia on espionage charges since December.

"We have protested to the MFA Paul Whelan’s mistreatment while in custody," wrote Andrea Kalan, the spokeswoman for the U.S. Embassy in Moscow, in a tweet Wednesday. The MFA is Russia's Ministry of Foreign Affairs.

The U.S. Embassy has called for an investigation into allegations of Whelan's treatment and a guarantee of Whelan's safety.

"Welfare of U.S. citizens abroad is our highest priority," wrote Kalan.

Whelan was arrested on December 28 in Russia for allegedly carrying out espionage, according to the Russia's Federal Security Service. Whelan's family asserts that he was in Russia for a wedding.  Whelan's lawyer, Vladimir Zherebenkov, alleges that his client received a USB drive that he had thought to be filled with photographs of his own vacation from a Russian citizen. He did not view the contents of the drive, according to his lawyer. The USB turned out to contain state secrets, including the names of members in Russia's security services.  

Zherebenkov told reporters in January that "how he got [the flash drive], what he was supposed to do with it, and whether Whelan knew that he had secret information is unknown."
 
Whelan holds citizenship in Canada, Ireland, and Great Britain and served in the Marines, ultimately receiving a bad conduct discharge. He maintains that he was framed.

Whelan also alleges mistreatment, including denial of medical care and consular access.

Former American intelligence officers have said that it is unlikely that he was an intelligence agent, given his discharge history and his lack of diplomatic cover in Russia.

Others have speculated that the move to arrest Whelan serves as a move on Russia's part to force a prisoner swap between him and Maria Butina, a Russian national and gun rights activist who plead guilty to acting as a foreign agent to influence U.S. politics.

Recently, in an appeal hearing, he directly called upon U.S. President Donald Trump to aid in his release.  "Mr. President, we cannot keep America great unless we aggressively protect American citizens wherever they are in the world," Whelan said in his hearing.